1. Field of the Invention
The invention relates to a conducting structure, and more particularly, to a conducting structure with flexibility.
2. Description of the Prior Art
In the fabrication of liquid crystal displays, some of the module package techniques common used today includes tape automated bonding (TAB), chip on glass (COG), and chip on film (COF). In TAB, numerous bumps are utilized for connecting a polyimide board and a liquid crystal display panel via an anisotropic conductive film (ACF) after the driving chip is fabricated. The bumps are essentially metal blocks comprising gold or lead-tin alloy on the bonding pad of the chip and during fabrication, the bumps are melted to connect the bonding pad and the circuits together. In COG, driving chips with fabricated bumps are connected directly onto an LCD panel by an anisotropic conductive film via a flip chip package. In COF, the electrical circuit originally designed for the printed circuit board is positioned on the polyimide board together with the driving chip.
In the past, TAB has generally been utilized for fabricating larger LCD panels whereas COG has been used for fabricating medium to small size LCD panels. However in recent years, COG has been widely used for fabricating LCD panels of various sizes in order to reduce cost of the panels. Nevertheless, numerous problems that ultimately affect production yield and quality of display still remain when the existing COG technique is applied to LCD panel fabrication.
Please refer to FIG. 1. FIG. 1 is a schematic view of the COG technique according to the prior art. By fabricating bumps 106 on the pad of an integrated circuit 102 and utilizing an anisotropic conductive film 108 as the interface, the integrated circuit 102 can be smoothly connected onto an LCD panel 104. In general, the bumps 106 are composed of materials such as gold or lead-tin alloy and the anisotropic conductive film 108 is composed of materials including numerous conductive pellets. By melting the conductive pellets between the bumps 106 and the anisotropic conductive film 108 via a hot embossing fabrication, the integrated circuit 102 can be electrically connected to a connection pad on the LCD panel 104. A high temperature (160to 190°C.) is normally required to carry out the hot embossing fabrication, and because of the fact that the heat expansion coefficient between the integrated circuit 102 and the LCD panel 104 differs significantly, an enormous amount of stress is often generated at the contact surface when the temperature returns to normal. As shown in FIG. 2, the resulting stress often causes a bending phenomenon on the integrated circuit 102 and the LCD panel 104. Eventually, the phenomenon would further induce a curtain mura and degrade the overall quality of the display.
In the prior art COG technique, the smoothness of the LCD panel 104 becomes particularly important as the integrated circuit 102 and the LCD panel 104 are composed of hard and rigid materials. In general, the smoothness of the LCD panels must be controlled to within a range of ±0.5 μm and the smoothness requirements for larger LCD panels are even stricter. Essentially, an LCD panel with poor smoothness often results in a connection failure between the integrated circuit 102 and the LCD panel 104 and ultimately decreases the overall product yield.
Moreover, since the anisotropic conductive film that includes numerous conductive pellets is commonly used for connecting the integrated circuit 102 and the LCD panel 104, maintaining a safe distance between the bumps 106 therefore becomes a critically important matter for preventing a short circuit. Please refer to FIG. 3. FIG. 3 is a schematic view of the connection between the bump 106, conductive pellets 108a, and the integrated circuit 102 and LCD panel 104. If the distance between the bumps 106 is too short and the conductive pellets 108a are concentrated in one area, a short circuit will be occurred between the bumps 106 and it will decrease the product yield. According to the prior art about COG technique, a distance of more than 15 μm between the bumps 106 is generally required in order to control the probability of a short circuit to be within an acceptable range. As a result, the contact area between the integrated circuit and the LCD panel needs to be maintained above a certain size and it is an unavoidable challenge to reducing the size of the contact area.